Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
2002-04-09
2004-03-02
Jenkins, Daniel J. (Department: 1742)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C420S425000, C420S901000, C419S003000, C419S008000, C419S041000, C419S046000
Reexamination Certificate
active
06699821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrically stable Nb
3
Al superconducting wires capable of supporting high critical current densities in applied magnetic fields above 10 Teslas at liquid helium temperature and methods for making the same.
2. Description of the Prior Art
The use of powder metallurgical techniques for the fabrication of Nb
3
Al superconducting wires and tapes is well known in the art and is documented in the technical literature. Examples include Akihama et al. [1], Thieme et al. [2], and Flukiger et al [3]. Powder metallurgical processes are only one of several techniques that have been developed in the art for Nb
3
Al superconducting wire and tape fabrication. Other processes include jelly-roll, rod-in-tube, and clad chip extrusion [4]. The aim of all these processes is to create a composite containing a fine structure of Nb and Al laminates. A reaction heat treatment is then applied to promote the formation of Nb
3
Al. In all cases, it has been found that the critical current density (J
c
) performance of these composites improves as the laminate size is reduced. Bormann et al. performed thin film experiments and found that optimum performance is to be achieved in these materials when the Nb thickness is less than 30 nm and the Al is in stoichiometric ratio (thickness of 9.2 nm for 30 nm Nb) [5]. Greater laminate thicknesses led to the formation of an inhomogeneous A15 phase and also to the formation of non-superconducting phases.
For conventional Nb—Al conductors, the reaction temperature is moderate—typically 800° C. Recently, it has been found that substantial enhancement in high field performance can be realized if much higher temperatures (>1950° C.) are employed. But, whereas the moderate temperature heat treatments are applied for times on the scale of hours, the high temperatures are applied for only seconds or even a fraction of a second. To control the time at temperature and to freeze the high temperature reaction product, the hot composite wire or tape is quenched in a bath of a low melting-point metal such as Ga. In order to obtain the desired A15 Nb
3
Al, the wire or tape is then heat treated at moderate temperature much as for conventional Nb—Al conductors. This process is referred to as “melt-quench/ordering heat treatment”, or more conveniently by its acronym “MQ-OHT”.
Melt-quenching of composites containing fine laminates of Nb and Al results in the formation of a supersaturated solid solution Nb—Al bcc phase. The ordering heat treatment (typically, 800° C. for 10 hours) then transforms the supersaturated bcc phase into fine grained, highly homogeneous A15 superconductor Nb
3
Al. Experiments involving MQ-OHT processing of Nb—Al composite wire are described in Buta et al [6]. EP1058321A2 is an example from the patent literature disclosing MQ-OHT processing of composites containing Nb and Al-alloy laminates.
Although MQ-OHT processed Nb
3
Al conductors presently display the best performance at high field, there are drawbacks to this approach. First, thick Nb sheaths are required in order to prevent wire-bursting due to pressures induced by the molten Al during melt-quenching. Such a sheath is wasteful in that it is ineffective as an electrical stabilizer and it contributes nothing to J
c
, reducing the overall current density capacity of the conductor, known as engineering current density, J
c
. Second, the temperatures involved in MQ-OHT are so high that copper cannot be used in any conventional way to stabilize the conductor, because the copper would simply melt. Since electrical stabilization is necessary for practical conductors, alternate means of applying the stabilizer, such as electro-plating must be employed, adding complexity and cost to the conductor fabrication process.
As for conventional Nb—Al conductors, it has been found that the performance of MQ-OHT conductors improves as laminate size is decreased. The dilemma for both approaches is that it is exceedingly difficult to produce practical lengths of multifilamentary conductor having the desired laminate sizes of less than 100 nm. For all fabrication routes, wire breakage becomes severe as this size range is approached. The primary cause is thought to be rapid hardening of the Nb—Al elements as a result of extensive cold-work and fine-scale inter-mixing of the Nb and Al. As the filament hardness increases, it eventually reaches a point where the other composite elements can no longer provide mechanical support, and the composite fails. The present invention discloses methods by which to circumvent this hardening problem for the production of high performance Nb
3
Al superconducting wire. The invention also permits the use of heat treatment temperatures below the melting point of copper while still achieving performance levels presently attainable only by higher temperature MQ-OHT processes.
In one embodiment of the invention, Nb and Al powders are mixed in stoichiometric ratio for Nb
3
Al, this powder mix is encapsulated in a copper or copper alloy tube, and the resulting composite is brought to multifilamentary wire by means of conventional wire processing and rebundling steps. The Nb and Al powders are characterized by a nanometer-scale particle size (<100 nm). Of relevance to this processing method is the analysis of Suryanarayana et al. [7] in which it is argued that a critical grain size exists for nanocrystalline materials below which Hall-Petch hardening is violated. Below this size, the bulk material begins to soften with decreasing grain size rather than harden.
In another embodiment of the invention, Nb—Al alloy powders are used in place of pure Nb and Al. These powders are preferably also nanometer in scale. The Nb—Al alloy consists of the supersaturated bcc phase characteristic of the MQ-OHT process. This phase is known to have only limited ductility. Of relevance to this processing method is the work of van Beijnen and Elen [8] on the fabrication of multifilamentary Nb
3
Sn superconducting wires by the powder-in-tube method. In this process, a powder precursor compound (NbSn
2
) is reacted with Nb in the encapsulating tube wall to produce superconducting Nb
3
Sn. The present invention is distinct from this prior art in that reaction of the powder cores or filaments with an adjacent material is unnecessary and, in fact, undesirable.
U.S. Pat. No. 4,411,959 and U.S. Pat. No. 4,575,927 disclose superconducting wires containing sub-micron powders of brittle superconducting materials and methods for making the same. The present invention is distinct from this prior art in that the Nb—Al alloy powders used are not themselves superconducting.
The nanometer-scale powders used in the practice of the invention may be fabricated by any of a number of available methods, including spark erosion, gas condensation, and electrodeposition. Several of these processes are discussed in an overview by Shaw [9].
SUMMARY OF THE INVENTION
It is an object of the present invention to provide methods by which to circumvent the core or filament hardness problems associated with breakage in the fabrication of Nb
3
Al superconducting wires. It is a further object of the invention to produce Nb
3
Al superconducting wire characterized by full electrical stabilization and high performance at high applied magnetic fields (>10 T).
The present invention utilizes powder metallurgical techniques in the fabrication of the Nb
3
Al superconducting wire. These include the steps of encapsulating powders in a ductile metal tube and processing the resultant composite to wire by means well known in the art, such as wire drawing. Multifilamentary composites are fabricated by way of rebundling single-core wires into metal tubes and processing the resultant multifilamentary composite to wire, as is also well known in the art.
Specifically, two methods are disclosed for the fabrication of Nb
3
Al superconducting wire. These methods may be summarized as follows:
1) Nb
Motowidlo Leszek R.
Rudziak Mark K.
Wong Terence
Composite Materials Technology, Inc.
Hayes & Soloway P.C.
Jenkins Daniel J.
LandOfFree
Nb3Al superconductor and method of manufacture does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Nb3Al superconductor and method of manufacture, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nb3Al superconductor and method of manufacture will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3245216